sebilite protector and Overking Bathezid drops this also. Ultra rare from Overking, i've seen more Vibrating Gauntlets of Infuse then EoT. I can also confirm Gorenaire.
I've played EQ for 11 years now and always notice when this one drops because of how rare it was back in the days. I have only seen it on the end mobs from the Kunark era and doubt some of the mobs listed here:
Sontalak - wrong era
froglok reet knight
froglok reet shaman
froglok reet wizard
Myconid Adept
Myconid Priest
myconid reaver
myconid spore king
myconid warrior
sebilite juggernaut
The Sebilis mobs listed above just don't drop the end game kind of spells like Emissary of Thule. There was a lot of rumors of rare raid mob spells dropping in lower Sebilis but after 70 real time days over the years in the zone I'm quite confident this is not the case.